System for operating one or more lasers to project a visible line onto a surface

ABSTRACT

At least one temporary visible reference line is projected onto a target surface, as for example, an athletic field, by several laser sources. A first laser source emits optical energy—at a wavelength of between 380 nm and 750 nm—along at least a first selectable path upon the target surface so as to form a temporary line thereon. A second laser source disposed at a second elevated, stationary location relative to the target surface different from the first stationary location emits optical energy along at least a second selectable path upon the target surface so as to form a temporary line thereon. In some instances, each laser light source may be capable of traversing the entire width of the target surface, in which case each laser source covers a discrete region. In other cases, respective laser sources arranged on opposite laterals sides of the target surface can be included so that segments contiguous with those projected by a corresponding one of the first and second laser sources can be generated to form a temporary line that does traverse the entire width of the target surface.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/385,219 filed on Mar. 10, 2003, now U.S. Pat. No. 6,796,041and entitled “SYSTEM FOR OPERATING ONE OR MORE SYNCHRONIZED LASERS TOPROJECT A VISIBLE LINE ONTO A SURFACE” and of U.S. patent applicationSer. No. 10/385,218 filed on Mar. 10, 2003, now U.S. Pat. No. 6,751,880and entitled “SYSTEM AND METHOD FOR OPERATING GROUPS OF LASERS TOPROJECT A VISIBLE LINE OF DEMARCATION ONTO DISCRETE REGIONS OF ANATHLETIC FIELD”, both of which are continuations-in-part of co-pendingU.S. patent application Ser. No. 10/320,304 filed on Dec. 16, 2002 andentitled “SYSTEM AND METHOD FOR DYNAMICALLY MARKING ATHLETIC FIELDSUSING A HANDHELD USER INTERFACE”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the projection of visible lines ontosurfaces upon which persons are standing (e.g., athletic fields duringsporting or entertainment events) and, more particularly, to systemsemploying laser light beam sources to generate such visible lines.

2. Discussion of the Background Art

In the game of football, a key objective of the team in possession ofthe ball (i.e., the “offense”) is to retain possession of that ball bymoving it far enough down the field. Specifically, the offense is givena set of four plays or “downs” to advance the ball by at least tenyards. Each time that distance is reached or exceeded, the offense issaid to have crossed a “first down” line, a new set of downs is earned,and the offense is allowed to continue its advance toward the goal lineof the opposing team (i.e., the “defense”). If the offense falls short,however, possession is lost and the two teams reverse their roles. Aregulation football field has a length of 100 yards and 53 yards. Thus,by way of example, a team gaining possession of the ball at its own 20yard line must move the ball a total of eighty yards in order to reachthe end zone of the opposing team.

In numerous occasions throughout an average football game, the officialsof the game must resort to sideline markers to establish whether theoffense has advanced the ball by the required distance. The standardalignment system that is utilized is generally a pair of poles connectedby a 30 foot long chain. The relative position of the football ismeasured by locating a first of these poles at the approximate locationof the initial line of scrimmage and moving the second as far forward aspossible. Each time this measurement is made, the game must be delayedand the yard markers must be carried from the sidelines to the place onthe field where the official has “spotted” the ball. Although the gameof football has become a relatively complex sport, involving literallyhundreds of millions of invested dollars, this time consuming system hasremained relatively the same since the conception of the sport.

A number of approaches intended to ameliorate the aforementioneddeficiencies have been proposed over the years, but none of them has metwith any degree of commercial success. U.S. Pat. No. 3,741,662, entitled“VISIBLE LINE MARKER” and issued to Pioch on Jun. 26, 1973, U.S. Pat.No. 3,752,588, entitled “LASER FOOTBALL FIRST DOWN MEASURING DEVICE” andissued to Chapman on Aug. 14, 1973, and U.S. Pat. No. 4,090,708 entitled“APPARATUS FOR MARKING FOOTBALL FIELDS” and issued to McPeak on May 23,1978. Each of the aforementioned patents involve the use of lasers forthe purpose of marking visible lines of demarcation on an athleticfield. One of the principal drawbacks of these systems is thetime-consuming and tedious method of operation.

Both Chapman and Pioch involve the use of track mounted, slidingprojectors that are located at the sidelines and several feet above thefield level. The lasers are mounted for oscillation in a vertical planeand the projected, low intensity beam developed by each must strike thefield at points of reference lying on an imaginary line of demarcationdefined by the intersection of the vertical plane with the fieldsurface. Accordingly, it is necessary for the operator to manuallyposition the projector for each reference point established. Like Piochand Chapman, McPeak discloses the use of a laser assemblies adapted toaccommodate sliding movement along the sidelines of a football field.McPeak, however, teaches that two oppositely directed beams should beaimed at a level above (i.e., “adjacent and parallel to”) the fieldsurface.

Another drawback associated with the aforementioned systems is that thelow-intensity output of these lasers is far too low to be visible by theplayers, let alone by an audience in, for example, a stadium setting.Indeed, the aforementioned systems are intended for use only in making afirst down measurement determination after each close play. As it turnsout, players intent on getting the ball past the first yard line—andfocused on the sideline markers long enough to be “blindsided” by thedefense —have suffered very serious neck and back injuries.

Television networks have recently implemented an image pre-processingsystem which allows viewers of televised football games to see aso-called “virtual” first down line that digitally projects, in realtime, a visible line onto video frames recorded by the televisioncamera, the line being displayed on a viewer's television set so that itappears to extend between the first down sideline markers.Unfortunately, neither the players, game officials, nor the fansattending such games can actually see this virtual line. It is thusreasonable to conclude that given the rapid and widespread adoption of avirtual visible line marking system—whose enjoyment is strictly limitedto television viewers, it has heretofore been deemed impossible orimpracticable to project a real, visible line onto the grassy surfaceswhich characterize most athletic fields. The individual blades of grasscomprising such fields tend to be randomly oriented and, for thisreason, will scatter incident light in many different directions. Inco-pending U.S. patent application Ser. No. 10/385,218, the inventorsherein described a laser-based line generating approach which addressedthe poor light scattering properties of grassy surfaces by operating twolaser-based laser light sources—a first laser light source on onelateral side of the field and a second light on the opposite lateralside of the field—to generate a line which extends across at least aportion of the field. Because at least two light beams strike each pointalong the desired region of the field, there is a greater likelihoodthat enough light will be reflected that the line so generated will beclearly visible (whether by a camera or a spectator) from the widestrange of viewing angles.

In certain installations, however, the inventors herein have found thatsatisfactory results can be achieved without using two oppositelysituated sources to generate a single line of demarcation (or segmentthereof). That is, under certain circumstances, light incident from onlyone side of the field is sufficient to afford viewing from the desiredrange of viewing angles. The inventors herein have further encounteredinstallations where the topological characteristics of the field surfaceare such that a laser line source positioned at one side of the targetregion can not uniformly traverse the entire width of the field. Forexample, many outdoor football fields are designed with an elevatedcentral region that causes rain to quickly drain to either side where itcan be safely carried away by a system of drainage canals.

A continuing need therefore exists for a visible line marking systemthat is simple to operate, accurate enough to allows its use byofficials at sporting events, and of sufficient intensity to be viewedby players, large audiences, and television viewers alike.

A need also exists for a system capable of projecting, onto surfacesthat are characterized by non-uniform topological features, a line whichcan be seen from different perspectives and from considerabledistances—even in daylight conditions.

A need also exists for a system that is capable of projecting a linewhich, though intense enough to be seen from a wide range of viewingangles, conforms to all applicable eye-safety regulations such as thosepromulgated by the FDA's Center for Diagnostic and Radiological Health(CDRH).

SUMMARY OF THE INVENTION

The aforementioned needs are addressed, and an advance is made in theart, by an apparatus for providing at least one temporary visiblereference line on a target surface, as for example, an athletic field.The system comprises a first laser light source disposed at a firstelevated, stationary location relative to the target surface, the firstlaser source being operative to emit optical energy—at a wavelength ofbetween 380 nm and 750 nm—along at least a first selectable path uponthe target surface so as to form a temporary line thereon. The systemfurther comprises a second laser light source disposed at a secondelevated, stationary location relative to the target surface anddifferent from the first stationary location, the second laser lightsource being operative to emit optical energy—at a wavelength of between380 nm and 750 nm—along at least a second selectable path upon thetarget surface so as to form a temporary line thereon.

In accordance with a first illustrative embodiment of the presentinvention, each of the first and second laser light sources comprises acorresponding projecting module that includes a cylindrical lensdimensioned and arranged to distribute the emitted optical energy alonga respective selectable path. To project a line or line segment havingsufficient intensity to be seen from the seats of a stadium, the outputbeams of a centralized group of lasers is collimated and opticallycoupled to either projecting module using an optical fiber.Advantageously, a group of pulsed lasers arranged in this fashion can beenergized sequentially to collectively develop a quasi-cw output that iscompliant with Laser Group I or IIIA operation. Class I operationrequires a laser source to develop a continuous output for at least 0.25seconds. Thus, by way of illustrative example, five pulse-mode laserswith a pulse width of 60 ms can be sequentially operated to collectivelygenerate a quasi-cw output exceeding the 0.25 second threshold.

Substantial optical energy is lost when the optical output of severallasers are coupled together in this fashion. As such, it may bedesirable to utilize a dedicated group of lasers for each of the firstand second laser light sources. In either event, the optical energyso-generated is directed through a cylindrical lens that spreads eachbeam into to a line that spans across at least a portion of the width ofthe target surface. Depending upon the topography of the target surface,it may be desirable to employ a third laser light sources disposed at alateral side of the target surface directly opposite to the first laserlight source and a fourth laser light source disposed at a lateral sideof the target surface directly opposite to the second laser lightsource. As will be readily appreciated by those skilled in the art,complementary pairs formed by the first and third laser light sources,and the second and fourth laser light sources may be operated so as tooverlap so as to create a more intense line that can be seen better froma wider range of viewing angles. Alternatively, these complementarypairs may be operated to form contiguous line segments along acorresponding selectable path. The latter operation has been found to bean especially useful way to address the problem of topologicalirregularities (e.g., the central “bulge” found on many footballfields).

It is expected that the power delivery requirements for each lasersource can vary considerably for each installation, depending upon suchvariables as the range of expected ambient lighting conditions, thedistance emitted optical energy must traverse before contacting thetarget surface, and the actual width dimension of the line to bedisplayed. For a line width of approximately 4 to 6 inches (10 to 15cm), excellent results have been achieved from distances in excess ofseveral hundred feet using 40 W, frequency doubled, Q-switched Nd:YAGlasers each adapted to generate laser pulses at a wavelength of 532 nm.Emission at this wavelength is especially preferred since it is veryclose to the peak (555 nm) of the eye's sensitivity. By comparison, inan argon laser operating in continuous wave (cw) mode, roughly half ofthe output is at 514 nm (58% as bright as the same beam at 555 nm),another 30% is at around 480 nm (18% as bright) and the remaining 20% isat around 440 nm (barely visible to he human eye). Thus, such an argonlaser would have to deliver up to three or four times as much power tomatch the visibility of the Nd:YAG laser. Notwithstanding the relativedifference in visibility, the inventors herein contemplate that one ormore cw-mode lasers can be used in conjunction with one or more pulsemode lasers to provide a single, composite visible line, if desired.Moreover, and with particular regard to an illustrative embodiment thatuses two laser sources to generate each visible line, it is alsocontemplated that the first laser source may be configured to deliver abeam which has a different power level than the second laser source, andthat the respective power levels may be altered as necessary tocompensate for changes in ambient lighting conditions.

In accordance with a modified embodiment of the invention, theprojecting modules may employ scanning technology—rather thancylindrical lenses—to generate a line along each correspondingselectable path. To move the respectively generated laser beam emittedby each group of lasers along a selectable path on a target surface soas, for example, to produce a straight line connecting the two lateralsides of a football field, each projecting module according to themodified embodiment includes a scanning assembly capable of deflectingthe beam as required to generate the composite line. By way ofillustrative example, the scanning assembly for each laser source mayinclude an X-Y galvanic scanner. The controller is responsive to userinput commands to move the composite temporary line from a firstselectable path on the surface to a second selectable path on thesurface. Where the installation is to provide a line of demarcation in afootball game, this can include moving the temporary visible lineforward from a first down line of scrimmage—where an official has just“spotted” the ball—by a distance of ten yards to thereby display the newfirst down line. This can also include moving the temporary visible linefrom an old line of scrimmage, forward or backward, to a new line ofscrimmage as a result of a penalty assessed against one of the teams.

Under certain ambient lighting and other installation conditions, it iscontemplated that a surface may be divided into multiple regions orzones. This allows the distance over which each beam must travel to bekept within a range that is consistent with the intensity, divergenceand line width demands for proper viewing. By way of illustrativeexample in which the surface is a football field, the first and thirdlaser light sources may be positioned above opposite lateral sides ofthe 25 yard line on one-half of the field, while second and fourth lasersources may be positioned opposite lateral sides of the 25 be yard lineon the other half of the field. It is considered within the level ofskill of the ordinary artisan to obtain, whether empirically or bycalculation, a juxtaposition of laser sources that is ideally suited tothe specific conditions of any indoor or outdoor location.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by reference to the detaileddescription of the invention that follows, taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a perspective view of a football stadium equipped with avisible line marking system in accordance with an illustrativeembodiment of the present invention;

FIG. 2 is a partial perspective view of the football stadium of FIG. 1,depicting the projection of a visible line of demarcation (i.e., a“first down” line) onto a portion of the field surface covered by realor artificial grass;

FIG. 3 is a block diagram schematically depicting the components of anexemplary visible line marking system employing two pairs ofsynchronized coherent laser sources;

FIG. 4 is a block diagram depicting, in greater detail, the variousfunctional elements of the exemplary visible line marking system of FIG.3;

FIG. 5 is a flow chart depicting a sequence of operation for theexemplary system depicted in FIGS. 3 and 4;

FIG. 6 is a block diagram schematically depicting the components of analternate embodiment of a visible line marking system; and

FIG. 7 is a block diagram depicting, in greater detail, the variousfunctional elements of the alternate visible line marking system of FIG.6.

DETAILED DESCRIPTION OF THE INVENTION

In connection with the exemplary football stadium installation depictedin FIGS. 1 and 2, it will be understood that the term “target surface”refers to the surface of an athletic field that is entirely orsubstantially covered by real or artificial turf grass. By appropriatebeam wavelength, output power level selection, and/or placement ofmultiple laser sources, the poor light scattering performance of suchnon-smooth surfaces can be overcome so that spectators can easily seethe line(s) so-projected—from most, if not all, vantage points withinthe seating area of a stadium or arena—even in peak daylight ambientlighting conditions. It should thus be understood that although theexemplary embodiments illustrated and described herein relatespecifically to the projection of a visible straight line onto the grasssurface of a football field, the teachings of the present invention areequally applicable to the projection of other types of lines—includingimages, logos, advertising messages, and the like—onto any surfacecovered by real or artificial turf.

FIG. 1 is a partial view of an exemplary stadium 100 having associatedtherewith a visible line marking system constructed in accordance withthe teachings of the present invention. In the center of stadium 100 isan athletic field 102 covered with grass—which can be either real orartificial turf grass—and marked with a rectangular grid pattern todefine a football playing area. The width of this grid pattern isdelineated by, inter alia, first and second lateral boundary linesindicated generally at 103 a and 103 b, which are separated by adistance of approximately fifty-three yards. At regular increments often yards, eleven transverse reference lines extend across field 102,interconnecting first and second boundary lines 103 a and 103 b.Collectively, these transverse reference lines define the one hundredyard area of field 102 that separates the end zones 108 of each team.

Surrounding the grass-covered surface of football field 102 is a seatingarea, indicated generally at 104, designed to accommodate a large numberof spectators. As will be readily appreciated by those skilled in theart, the seating area of a typical professional league football stadiumcan easily accommodate several scores of thousands of fans, and manycollege arenas provide seating for at least tens of thousands. In thatregard, seating area 104 can consist of three or more distinct tiers as,for example, a lower deck, mezzanine, and upper deck area. Betweenseating area 104 and playing field 102 is a retaining wall 106, whichserves as a barrier between the spectators and the players and officialson field 102. It goes without saying that the spectators expect asubstantially unobstructed view, from any seat within seating area 104,of the action taking place on field 102.

A line marking system constructed in accordance with an illustrativeembodiment of the present invention includes a first pair of lasersources indicated generally at 120 a and 120 b and a second pair oflaser sources indicated generally at 130 a and 130 b. To ensure coverageof the entire length and width of the playing area, each laser source ispositioned at a location that is high above the grass-covered surface offield 102—on the order of from about fifty to about two hundred andfifty feet or so depending upon the intensity, shape and divergence ofthe coherent beam generated and upon the availability of a suitablemounting location. Although it is conceivable that certain enclosed(e.g., domed) environments might offer a mounting location that isdirectly above athletic field 102, each laser source as sources 120 aand 120 b is typically mounted well beyond lateral boundary lines 103 aand 103 b-on the order of, say, about fifty to one hundred and fiftyfeet outside lines 103 a and 103 b. In the exemplary embodiment of FIG.1, for example, each laser source is positioned directly above seatingarea 104, with care being taken to ensure that the respective beamsprojected are sufficiently distant from the spectators at all times asto comply with the guidelines prescribed by the Center for Diagnosticand Radiological Health, a department of the U.S. Food and DrugAdministration. The beam may therefore traverse a distance of hundredsof feet before reaching the surface of field 102, and may do so at anangle of incidence that is typically within a range of from aboutfifteen to about ninety degrees.

Under certain ambient lighting and other installation conditions, it iscontemplated that a surface may be divided into multiple regions orzones. This allows the distance over which each beam must travel to bekept within a range that is consistent with the intensity, divergenceand line width demands for proper viewing. By way of illustrativeexample in which the surface is a football field that is subject todaylight illumination conditions, first and second laser sources 120 aand 120 b may be positioned above opposite lateral sides of the 25 yardline to provide coverage for half the area of field 102— a regiondesignated as area 102 b in FIG. 3, while third and fourth laser sources130 a and 130 b may be positioned opposite lateral sides of the 25 beyard line on the other half of the field, a region designated as area102 a in FIG. 3. Such an installation decreases the maximum angle,relative to the vertical, at which each beam strikes field surface 102.A beam emitted by laser source 120 a from a point 200 feet above alateral side of the 25 yard line will be disposed entirely in a verticalplane relative to horizontal target surface 102 b, and thus at an angleof zero degrees relative to the vertical. A beam directed from that samesource but along a selectable path 25 yards away (e.g., at the 50 yardline) will strike target surface at an angle of twenty degrees relativeto the vertical. To be useful as an official line of demarcation in afootball game, it is believed that the angle should be no greater than20 degrees. This is because the tip of the football is three inchesabove the ground. In the foregoing example, the trailing edge of thevisible composite line will cross the tip of the football about one inchin front of where it actually crosses the field. The greater the greaterthe angle, the greater the deviation.

As such, and in accordance with the illustrative embodiment of thepresent invention depicted in FIG. 1, each pair of laser sources isdimensioned and arranged within stadium 100 to provide coverage for onlya portion of the entire of the field area. As best seen in FIGS. 2 and3, laser sources 120 a and 120 b—in a manner to be described shortly—areoperated together so as to jointly project, from two different angles, acomposite visible line 110 onto the field region 102 b. As indicatedabove, by appropriate beam wavelength, output power level selection, andplacement of the multiple laser sources, the poor light scatteringperformance of the grassy field surface 102 can be overcome andspectators can easily see the line so-projected from most, if not all,vantage points within seating area 104—even in peak daylight ambientlighting conditions.

A simplified block diagram of an illustrative visible line markingsystem 10 constructed in accordance with the teachings of the presentinvention is shown in FIG. 3. Essentially, system 10 includes aplurality of groups of laser sources, of which only a first group(comprising first laser source 120 a and second laser source 120 b) anda second group (comprising third laser source 130 a and fourth lasersource 130 b) are shown. It will readily appreciated by those skilled inthe art that any number of intermediate groups of laser sources may beadded, and the relative spacing between the sources of all groupsadjusted, in order to ensure that the projected line 110 (FIG. 2) can beclearly seen from all desired viewing angles.

In this regard, the inventors herein have observed that the intensity oflight reflected by grassy surfaces is subject to substantial localvariations depending upon the vertical and angular position of theobserver relative to the location where a laser beam strikes a region ofthe target surface. In locations where the amount of ambientillumination is relatively high such, for example, a stadium whose grassfield is exposed to full sun or even bright incandescent lighting, lightprojected by a single coherent laser source is reflected by the randomlyoriented blades of grass in such a way that it can be clearly seen fromsome seating locations and barely seen from others. In accordance withthe illustrative embodiment of the present invention depicted in FIGS. 3and 4, the light from two or more optical sources, as scanned beam 124output by first laser source 120 a and scanned beam 122 output by secondlaser source 120 b, are used in order to ensure that the light reflectedby target surface 102 b can be clearly seen from any viewing location.

With continuing reference to FIG. 3, it will be seen that the respectivelaser sources are controlled by host computer system 30, with whichthere are associated a monitor 22, a keyboard terminal 36, and ahand-held wireless interface 42. In the exemplary embodiment, thesoftware controlling movements of the beams developed by each lasersource is configured to allow line projection position and operationcommands to be supplied by either keyboard terminal 36 or by wirelessinterface 42 (via an RF link). To make the most effective use of thecapacity to implement line position commands using keyboard terminal 36,monitor 22 is configured to present a view (which may be an actual viewtaken by video camera or a digitally simulated scene) of football field102. Alternatively, or in addition to the keyboard command capability,commands for position the visible line are entered by wireless interface42 via an encrypted RF link.

As will be readily appreciated by those skilled in the art, an advantageof the keyboard implementation is that it is can be o-located withmonitor more secured It will also be seen that field surface 102 (or anyportion thereof) is within the view of at least one broadcast videocamera indicated generally at 20. In accordance with the standard NTSCtelevision interlaced format, the view is scanned at a rate of 30 Hz.Typically, the scanning format is an array of 488 (H) by 380 (W) chargecoupled device (CCD) pixels, each of which generates a voltage levelproportional to the intensity of light on that pixel element. An NTSCcamera converts this sequence of pixel outputs to a standard RS 170composite video signal of 525 lines, two fields per frame, fullyinterlaced format with a resolution of 488 lines per frame by 380elements per line.

Turning now to FIG. 4, it will be seen that each laser source as source120 a comprises first and second galvanic scanners indicated generallyat reference numerals 121 and 123, respectively. Such scanners are foundin conventional laser projectors, and as used therein, galvanic scanner121 controls movement in the X-axis direction of a coherent laser beamdeveloped by beam generator 140. Likewise, galvanic scanner 123 controlsmovement in the Y-direction. Considering the width direction of field102 between boundaries 103 a and 103 b to constitute the Y-direction, avisible line is generated by causing beams 122 and 124 (FIG. 3) to movefast enough in that direction to create a composite temporary visibleline at a desired location on target surface 16.

Essentially, a composite visible line is formed at a desired location byrepeatedly and rapidly scanning target surface 102 with each of beams122 and 124 such that each beam strikes target surface 16 at many pointsalong a selectable path. An exemplary selectable path is identified byreference numeral 110 in FIG. 2, it being understood that a change inthe specific location of the temporary visible line—in this case astraight line across surface 102 b—is implemented through operation ofthe respective X- and Y— scanners of each of laser sources 120 a and 120b. As will be readily appreciated by those skilled in the art, eachscanner as scanners 121 and 123 includes mirror (not shown) thatdeflects the beam. Working together, scanners 121 and 123 are operativeto direct the corresponding beam at any selectable point within coverageregion 102 b so as to thereby generate a temporary visible line thereon.

To enable accurate positioning of a visible line along a selected pathas path 110 (FIG. 2), scanners as scanners 121 and 123 are preferablyclosed-loop galvanic scanners (also called “position detecting”scanners). Scanners of this type are commonly used in the laser lightentertainment industry and are compatible with a wide variety ofcommercially available laser graphics software packages. Acceptableperformance has been achieved using scanners that are capable ofdirecting the beam to 24,000 to 30,000 discrete points along selectedpath 110 every second. Scanning assemblies suitable for use in thepresent invention have been assembled, for example, using two ofCambridge Technology's model 6800 scanners and matching model 6580amplifier circuit boards.

As a safety precaution, each laser projector preferably includes aconventional shutter mechanism (not shown) such, for example, as anacoustic optical modulator (AOM) for turning off the beam in the event,for example, a malfunction prevents proper movement of each scanningbeam. In the event system 10 may be called upon to create two or moredistinct and unconnected visible lines, the AOM's may also be used toimplement a blanking function whereby the beam is turned off as it movesbetween them.

Optionally, each laser source may further include a conventional beamexpander (not shown) in order to increase the diameter of the beam or aconventional collimator (not shown) for altering its divergence. In atypical stadium installation, it is anticipated that laser sources assources 120 a and 120 b will be mounted anywhere from about 75 to about200 feet above the level of target field surface 102. Consequently,beams 122 and 124 will traverse a considerable distance before strikingsurface 102 b. As will be readily appreciated by those skilled in theart, the need for expansion or collimation of beams 122 and 124 ispurely a function of the initial beam diameter and the desired thicknessof the visible line as formed on the target surface. A more challengingaspect of projecting beams over such distances, especially in full sunillumination conditions, is that of finding lasers capable of deliveringcoherent beams of sufficient power and intensity to form a visiblecomposite line.

For a line width of approximately six inches (15 cm), excellent resultshave been achieved in a stadium environment (i.e., from distances inexcess of several hundred feet) using two 40 W, frequency doubled,Q-switched Nd:YAG lasers each adapted to generate laser pulses at awavelength of 532 nm. Emission at this wavelength is especiallypreferred since it is very close to the peak (555 nm) of the human eye'ssensitivity. By comparison, in an argon ion laser operating incontinuous wave (cw) mode, roughly half of the output is at 514 nm (58%as bright as the same beam at 555 nm), another 30% is at around 480 nm(18% as bright) and the remaining 20% is at around 440 nm (barelyvisible to he human eye). Thus, an argon laser would theoretically haveto deliver up to three or four times as much power to match thevisibility of the Nd:YAG laser. Notwithstanding the relative differencein visibility, the inventors herein contemplate that one or more cw-modelasers can be used in conjunction with one or more pulse mode lasers toprovide a single, composite visible line, if desired. Moreover, and withparticular regard to an illustrative embodiment that uses two lasersources to generate each visible line, it is also contemplated that thefirst laser source may be configured to deliver a beam which has adifferent power level than the second laser source, and that therespective power levels may be altered as necessary to compensate fordifferent ambient lighting conditions.

The use of Nd:YAG lasers has heretofore been regarded as unsuitable forso-called laser graphics applications because they tend to producedotted, rather than continuous lines. Advantageously, the use of two ormore lasers in accordance with the teachings of the present inventionovercomes this apparent deficiency by synchronizing the first and secondlaser sources such that segments of the broken pattern of ellipticalspots produced by first laser source 120 a overlap the broken areasbetween the elliptical spots produced by second laser source 120 b. Theresulting composite visible line appearing along selected path 110 thusappears to be continuous and unbroken to the human observer. If desired,a cylindrical lens can be used to define the appearance of each spot asa dotted line segment having a straight forward and trailing edge.

In any event, and with continuing reference to FIG. 4, it will be seenthat the scanners associated with each corresponding laser source arecontrolled by a corresponding laser projector control modulerespectively identified by reference numerals 82 a, 82 b, 82 c and 82 dresiding within host computer system 30. Acceptable results have beenobtained using Pangolin QM2000 laser projection controller boards,available from Pangolin Laser Systems, Inc., Orlando, Fla. Essentially,each QM2000 board includes its own processor and memory storageresources, and is configured to execute a special software program(Pangolin LD2000) to directly control any ILDA-compliant scanner unit. Afirst of the projector control modules, control module 82 a, isdesignated as a “master” controller and is configured to assign specificline projection tasks to the scanners (e.g., 121, 123) of its ownassociated laser source as well to those of the laser sources via“slave” projector control modules (e.g., 82 b-82 c). Utilizing thePangolin LD2000 software package, it is possible to define a series of“scenes” each corresponding to a discrete position of the visible lineto be projected.

Other components of host computer 30 include a conventional centralprocessor unit as, for example, an Intel Pentium 4 2.0 GHzmicroprocessor unit, random access memory 86, a hard drive for storageof the operating system and communications program needed to define aninterface between wireless user interface 42 and I/O ports 90 via radiofrequency (RF) transceiver 43. A set of MIDI function commands input bylocal console 32 or wireless, handheld user interface 42 cause theprogram executing on master projector control module 82 a to instruct anappropriate group of scanners to move the beams as needed to adjust thevisible, composite temporary line from an initially selected positiondefined by a first “scene” stored in RAM of module 82 a andcorresponding to a selectable path as path 110 in FIG. 2, to asubsequently selected position defined by a second scene. Thus, forexample, in the context of an illustrative football stadiuminstallation, the temporary line may be moved from an initial line ofscrimmage—where a game official has just “spotted the ball”—by a setdistance of ten yards by the mere depression of a single pushbutton ofuser interface 42. This can also include moving the temporary visibleline from an old line of scrimmage, forward or backward, to a new lineof scrimmage as a result of a penalty assessed against one of the teams.

From the foregoing discussion it will be appreciated that system 10, asthus far described, is capable of creating temporary, visible compositelines which can be seen not only by players and game officials on theground, but also by stadium audiences from distances in excess ofhundreds of feet—despite the non-uniform light scattering properties ofreal and artificial grass. So long as the sweep frequency rate—the rateat which the visible line is refreshed by passing the beams across it—isat least thirty times per second, no flicker will be perceptible by theplayers, officials, or spectators present at the event. In order for thelines generated by system 10 to be properly seen by televisionaudiences, however, it is necessary to synchronize the beam scanningprocess to the scan rate of the at least one television or video camera20 having surface 102 within its field of view.

Assuming NTSC compliant operation, the sweep frequency rate must be an nmultiple of the 30 Hz camera scan rate, where n is a whole integergreater than one (e.g., for n=2, the sweep frequency rate is 60 Hz).Since it is believed to be easier in most cases to trigger operation ofthe projector control modules rather than trigger the scan cycle of thebroadcast video or television camera, an especially preferred embodimentof the present invention includes a phase-locked synchronization circuit44 which synchronizes the operation of laser projection control modules82 a, 82 b, 84 a and 84 b to the 60 Hz vertical blanking pulses of thevideo signal from broadcast camera 20. For a 30 Hz refresh rate, the 60Hz vertical retrace output signal from camera 20 is coupled to a divideby 2 frequency divider (not shown).

With reference now to FIG. 5, an exemplary sequence of operating theillustrative visible line marking system depicted in FIGS. 1-4 will nowbe described. The process is entered at step 202 wherein a determinationis made as to whether an n Hz synchronization signal is available froman external source such, for example, as a broadcast television camera.In most cases, the reference signal will be a 30 Hz or 60 Hz signal,though other n-multiples of 30 Hz are contemplated. If no referencesignal is available, one is generated at step 204. Each laser projectorcontrol module is synchronized to the common reference signal so thatall laser sources can be operated to sweep their associated beam acrossthe field surface at the same precisely controlled rate (block 206).

Using a hand-held user interface, a game official enters commandsinstructing visible line system 10 as to the required location ofdynamically movable line 110 (FIG. 2). An RF link between host computer30 (FIG. 4) and this interface is continuously monitored for new lineplacement instructions (block 208). If a new line projection command isreceived (block 210), it is examined to determine whether it is aterminate command (block 212). If so, all line projection operationscease (block 214) and the process terminates. If not, the processproceeds to block 216 and a newly received line projection command issupplied to master projector control module. Using its own on boardprocessing and memory resources, the master projector control moduleobtains scene information corresponding to the location of the visibleline to be projected and an indication as to which slave projectorcontrol module(s) are required to operate the applicable laser sources(block 218). At block 220, sub-commands are supplied by the mastercontrol module to the appropriate slave projector control modules andthese, in turn, deliver data to the scanners associated with the lasersources controlled by them. The process returns to block 208. Until aterminate command or new line projection command is received, system 10projects the visible line at the location specified by the most recentprojection command.

Turning now to FIGS. 6 and 7, there is shown a further illustrativeembodiment of the present invention. In the embodiment depicted in FIGS.6 and 7, a plurality of sequentially pulsed Nd:YAG lasers whoserespective outputs are fiber fed into an optical combiner used to supplyone or more laser sources as sources 120 b and 130 b with the requisiteoptical energy. Although an arrangement in which two separate groups ofpulsed lasers are used to feed respective lateral groups of lasersources is shown, other arrangements are also possible. For example, thelasers may be consolidated into a centralized, environmentallycontrolled enclosure situated in such a way as to allow an optical fiberto feed each of laser sources 120 a, 120 b, 130 a and 130 b. Though eachindividual laser may be pulsed for only a fraction of the 0.250 secondduration needed to achieve Class I operation, when they are groupedtogether and pulsed sequentially a composite pulse can be generatedwhich satisfies the 0.250 second threshold needed.

Within each laser source, the X-Y scanning assembly is replaced by acylindrical lens arrangement which converts the fiber-fed beam sourceinto a line segment capable of spanning either a portion of the width ofthe field—in which case at least two contiguous segments are required orthe entire width of the field—in which case the lines may overlap (inthe manner shown in the arrangement of FIG. 6).

It will be readily appreciated by those skilled in the art that variousmodifications and enhancements are possible. For example, there is norequirement that any pair of laser sources, as first and second lasersources 120 a and 120 b, be located along a line transverse andperpendicular to the lateral sidelines of the field. Thus, for example,the first laser source might be outside the first lateral side of fieldregion 102 b at the twenty-yard line and the second laser source mightbe outside the second lateral side of region 102 b at the thirty-yardline. Still another laser source of the same group might be outside thesecond lateral side of region 102 b at the ten-yard line, such that allor any two laser sources of the group might be used to generate avisible composite line in accordance with the present invention.

Nor is their any requirement that the laser sources be located at thesame elevated vertical position relative to the field. It suffices tosay that it is considered to be within the level of skill of theordinary artisan to obtain, whether empirically or by calculation, ajuxtaposition of laser sources that is ideally suited to the specificlighting conditions and overall dimensions associated with anyparticular indoor or outdoor location.

It will be apparent to those skilled in the art that variousmodifications and variations can be 20 made to the present inventionwithout departing from the spirit and scope of the invention. Thus, itis intended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. An apparatus for providing a temporary visible reference line acrossa surface, comprising: a first laser source disposed at a firstelevated, stationary location relative to the surface, said first lasersource being operative to emit optical energy at a wavelength of between380 nm and 750 nm along a selectable path within a first region of thesurface so as to form at least a portion of a temporary visible linethereon; and a second laser source disposed at a second elevated,stationary location relative to the surface, said second laser sourcebeing operative to emit optical energy at a wavelength of between 380 mand 750 m along a selectable path within a second region of the surfaceso as to form at least a portion of a temporary visible line thereon;and a controller, said controller being responsive to input usercommands to operate said first laser source when a temporary visibleline is to be formed within the first region and to operate said secondlaser source when a temporary visible line is to be formed within thesecond region of the surface.
 2. The apparatus according to claim 1,wherein each said laser source comprises a line projecting moduleincluding a cylindrical lens.
 3. The apparatus according to claim 1,wherein each said laser source comprises an X-Y scanning assembly. 4.The apparatus according to claim 1, further including a plurality oflasers each operative to generate pulses and wherein said controller isoperative to sequentially operate at least some of said plurality oflasers so as to form a composite pulse having a duration of at least0.25 seconds.
 5. The apparatus according to claim 4, each of saidplurality of lasers is a frequency-doubled Nd:YAG laser emitting anoptical beam at a wavelength of 532 nm.
 6. The apparatus according toclaim 4, wherein said first laser source includes a first lineprojecting module having a cylindrical lens and the second laser sourceincludes a second line projecting module having a cylindrical lens, andwherein the output of said at least some of said plurality of lasers isoptically coupled to the line projecting module of at least one of saidfirst and second laser sources.
 7. The apparatus according to claim 4,wherein said first laser source includes a first line projecting modulehaving an X-Y scanning assembly and said second laser source includes asecond line projecting module having an X-Y scanning assembly andwherein the output of said at least some of said plurality of lasers isoptically coupled to the line projecting module of at least one of saidfirst and second laser sources.
 8. The apparatus according to claim 1,further including a third laser source disposed at a third elevated,stationary location relative to the surface, said third laser sourcebeing operative to emit optical energy at a wavelength of between 380 nmand 750 nm along a selectable path within the first region, and a fourthlaser source disposed at a fourth elevated, stationary location relativeto the surface, said fourth laser source being operative to emit opticalenergy at a wavelength of between 380 nm and 750 nm along a selectablepath within the fourth region, wherein said controller is responsive toinput user commands to operate said first laser source and said thirdlaser source when a temporary visible line is to be formed within thefirst region and to operate said second laser source and said fourthlaser source when a temporary visible line is to be formed within thesecond region of the surface.
 9. The apparatus according to claim 8,wherein said controller is operative to cause said first laser source toproject a temporary line segment that is aligned and contiguous with atemporary line segment projected by the third laser source so as tothereby form a continuous, temporary visible line, and to cause saidsecond laser source to project a temporary line segment that is alignedand contiguous with a temporary line segment projected by the fourthlaser source so as to form a continuous, temporary visible line.
 10. Theapparatus according to claim 9, wherein a continuous temporary lineformed within the first region by said first and third laser sources andwithin said second region by said second and fourth laser sources is astraight line having a length of about fifty three yards and a width offrom about four to about eight inches.
 11. The apparatus according toclaim 8, wherein said controller is operative to cause said first lasersource to project a temporary line segment that overlaps a temporaryline segment projected by the third laser source so as to thereby form acomposite, temporary visible line within the first region, and to causesaid second laser source to project a temporary line segment thatoverlaps a temporary line segment projected by the fourth laser sourceso as to form a composite, temporary visible line within the secondregion.
 12. The apparatus according to claim 11, wherein a compositetemporary line formed within the first region by said first and thirdlaser sources and within said second region by said second and fourthlaser sources is a straight line having a length of about fifty threeyards and a width of from about four to about eight inches.
 13. A methodfor providing a temporary visible reference line across a surface,comprising the steps of: receiving a first command to generate atemporary visible line within a first region of the surface; operating afirst laser source operative to emit optical energy at a wavelength ofbetween 380 nm and 750 nm along a selected path within the first regionof the surface; receiving a second command to generate a temporaryvisible line within a second region of the surface; de-energizing thefirst laser source so as to terminate generation of a temporary visibleline within the first region and operating a second laser sourceoperative to emit optical energy at a wavelength of between 380 nm and750 nm along a selected path within the second region of the surface.14. The method of claim 13, wherein a third laser source is operatedduring the step of operating the first laser source.
 15. The method ofclaim 13, wherein a fourth laser source is operated during the step ofoperating the second laser source.
 16. The method of claim 13, whereinsaid step of operating a first laser source includes sequentiallypulsing a first plurality of lasers to generate a composite pulse havinga pulse width of at least 0.25 seconds.
 17. The method of claim 16,wherein said step of operating a second laser source includessequentially pulsing the first plurality of lasers to generate acomposite pulse having a pulse width of at least 0.25 seconds.
 18. Themethod of claim 16, wherein said step of operating a second laser sourceincludes sequentially pulsing a second plurality of lasers to generate acomposite pulse having a pulse width of at least 0.25 seconds.